1. Field of the Invention
The present invention relates to a magnetic recording medium and more particularly to a magnetic recording medium which can realize high-density recording by virtue of high coercive force and, in addition, has an improved S/N ratio by virtue of a reduced medium noise level while maintaining a highly reproduced output. The present invention also relates to a magnetic disk device, for recording and reproducing information, using the magnetic recording medium.
2. Description of the Related Art
The development of information processing techniques has led to an increasing demand for an increase in density of magnetic disk devices used in external storages of computers. Specifically, in the reproducing head of the magnetic disk devices, the use of a magnetoresistive head utilizing a magnetoresistor, or MR head, has been proposed instead of the conventional wire wound-type inductive thin film magnetic head. In an MR head, the electric resistance changes in response to the magnetic field intensity. The MR head is also characterized by magnetoresistance effects, which are changes in electric resistance produced in a magnetic material upon application of an external magnetic field, for the reproduction of a signal on a recording medium. An MR head also has a reproduction output margin that is several times larger than that of the conventional inductive thin film magnetic head. In the conventional head, the inductance is small and a large S/N ratio can be expected. Further, the use of an AMR (anisotropic magnetoresistive) head utilizing anisotropic magnetoresistance, a GMR (giant magnetoresistive) head utilizing giant magnetoresistance, and a practical variant, a spin valve GMR head, besides the MR head, have also been proposed.
Further, in order to meet the demand for high-density recording, a sufficient improvement in properties of a MR head, AM head, or GMR head (including spin valve head) have been demanded. In particular, low tBr (a product of the thickness t and the residual magnetization density Br of a magnetic recording layer), low noise level, and high coercive force Hc are required of the magnetic recording medium. In order to meet such requirements, the prior art, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-256017, discloses a magnetic recording medium comprising a nonmagnetic substrate, a chromium layer (thickness=100 nm) as an underlayer provided on the nonmagnetic substrate, and a magnetic layer (thickness=60 nm) provided on the chromium layer, the magnetic layer being formed of a CoCrTaPt-base four-component alloy and having a combination of a low noise level typically derived from CoCrTa-base alloy with high coercive force typically derived from CoCrPt-base alloy. U.S. Pat. No. 5,004,652 (corres. to Japanese Unexamined Patent Publication (Kokai) No. 4-228105) discloses a magnetic recording medium comprising a nonmagnetic substrate and a chromium layer (thickness=about 30-300 nm) and a magnetic layer of a CoCrPtTa four-component alloy (thickness=about 20-100 nm) formed in that order by sputtering on the nonmagnetic substrate. Further, Japanese Unexamined Patent Publication (Kokai) No. 5-72016 discloses a process for producing a magnetic recording medium, wherein a magnetic metallic thin layer is sputtered on a substrate. The above patent publications teach useful means for enhancing the coercive force of a magnetic recording medium. However, no consideration is given to a reduction in noise level which is one of the objects of the present invention.
Japanese Unexamined Patent Publication (Kokai) NO. 7-50008 teaches a magnetic recording medium which can simultaneously satisfy both property requirements of high coercive force and low noise level. Specifically, the magnetic recording medium taught in this publication comprises a nonmagnetic substrate layer and a magnetic layer provided on the nonmagnetic substrate layer through a nonmagnetic metal underlayer (thickness=100 to 3000 .ANG.) of chromium or a chromium alloy, the magnetic layer being formed of an alloy containing 60 to 80 at % cobalt, 5 to 20 at % chromium, 1 to 20 at % platinum, and 0.5 to 6 at % at least one member selected from the group consisting of niobium, hafnium, tungsten, titanium, and tantalum. According to this invention, not only a high coercive force of 1610 to 1750 Oe (Examples 1 to 7) but also a low noise level can be achieved. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-50009 discloses a magnetic recording medium wherein a thin layer medium of an alloy of 95 to 60 at % chromium and at least one member, in an amount of 5 to 40 at %, selected from molybdenum and tungsten is used as an underlayer for a magnetic layer having a single layer structure of a CoCrPt alloy. This magnetic recording medium also can realize simultaneously high coercive force and a low noise level. Specifically, the use of a chromium layer containing 28 at % molybdenum as the underlayer results in about 10% reduction in noise as compared with an underlayer consisting of chromium alone. The techniques taught in these publications, however, have problems remaining unsolved. For example, the magnetic recording medium described in Japanese Unexamined Patent Publication (Kokai) No. 7-50008, as specifically demonstrated in working examples, can provide higher coercive force (Hc.gtoreq.1500 Oe) than the comparative magnetic recording medium. The coercive force, however, is in the range of from 1610 to 1750 Oe at the highest. It is therefore desired to provide a magnetic recording medium which can provide coercive force Hc.gtoreq.2000 Oe. Regarding tBr, the magnetic recording medium described in the above publication, as demonstrated in working examples, provides a value in the range of from 320 to 340 G..mu.m which is unsatisfactory for coping with an increase in density a higher level of which is expected to be demanded in the future. It is therefore desired to provide a magnetic recording medium which has a tBr of 200 G..mu.m at the highest